curveball

Reader's Digest 10月号の記事 'Anatomy of a Perfect Hand Transplant' からの抜粋です。
At 3:32 p.m., the first cut was made to prepare Jonathan's arm. Dr. Azari arrived within the hour and joined his team. The first curveball came right away. The doctors had planned to sever the radius and ulna bones at about 11 centimeters above the wrist. But after opening up Jonathan's arm, preserving more bone seemed possible.
"curveball" はもちろん野球用語から来たのでしょう。物事は素直に真っすぐに進むことの方がむしろ少ないとも感じています。辞書での説明を見ます。
・Oxford English Dictionary: Something which is unexpected, surprising, or disruptive.: there's always a curveball on every mission
・Wiktionary: An unexpected turn of events initiated by an opponent or chance. Life has thrown him a few curveballs.